Subjecting exposed photographic film to a "fixing" solution removes the silver in the film that has not been converted by exposure to light and the subsequent development process. This silver accumulates in the solution, and is removed both to maintain the activity of the solution, and to recover the value of the silver. The common recovery process is based on electrolysis, and is similar to electroplating. The process must be carefully controlled to prevent some of the plating current from involving the thiosulfate ions present in the solution, and contaminating the deposited silver with a silver sulfide. This is usually accompanied by the objectionable release of hydrogen sulfide gas.
Several arrangements have been devised to control the plating current with enough precision that excesses do not develop beyond the carrying capacity of the silver concentration in the solution. Obviously, the plate-out procedure progressively decreases this concentration. In addition, a number of other variables have pronounced effects on the carrying capacity of the solution. These include temperature, solution level, and electrode area, in addition to the variables introduced by the control system itself. The principles on which control is based have included control as a function of the following:
(a) The color of the deposited silver. PA1 (b) Voltage change as plating proceeds. PA1 (c) Independent voltage-monitoring by sensor. PA1 (d) Threshold voltage required to pass a current through the solution, and the current induced by this threshold voltage (with an independent detector circuit). PA1 (e) Measurement of decay time, in which current is removed after a known voltage has been suppressed for a particular period. The residual cell voltage is monitored for a fixed period of time.
United States Patents illustrative of some of the above approaches include U.S. Pat. Nos. 3,551,318; 3,751,355; and 3,875,032. The British Pat. No. 1,144,756 (1969) has also been noted.
All of these systems appear to have characteristic problems associated with them. Where the system assumes a known voltage versus current relationship for varying silver concentration, the temperature dependency of the fixing solution is usually not accounted for. Solution levels, cathode area, and anode area are critical. Monitoring of current in a DC path is difficult at high plating currents, to the necessary accuracy. Electrical connection voltage drops are also critical, and are difficult to control, especially so in cases where disengageable connections are used in portions of the circuitry. Additionally, the plating process causes increased cathode surface area, changing the plating characteristic in the positive feedback manner. Where rotating cathode or anode are used in a system, the resulting commutation voltage drops present an additional unpredictable variable. Where a separate sensor is used to detect the concentration of the solution, and monitor the plating voltage accordingly, temperature dependency is also usually not accounted for. The surface of the sensor must be cleaned regularly to remove deposited silver, or sulfiding occurs. Other contaminants on the sensor surfaces also have the effect of reducing surface area, causing wasted silver due to reduced sensor currents at a fixed silver concentration level. Beyond this, the sensor system itself is an additional cost factor. In systems based upon the measurement of decay time, it has been found that the configuration has poor anti-sulfide characteristics in the low-current state. Temperature compensation is also not usually applied, and the recovery of silver is not continuous. The high current state in this system can also occur in a sulfiding condition. These problems have resulted in the development of the present invention.